In recent years it has been realised that the clearance between the tips of rotor blades and their associated static shrouds has a significant effect on the efficiency of operation of the stage of blades in question. Various attempts have therefore been made to maintain as small a clearance as possible in this situation. These attempts have largely involved modification of the static shroud to enable the internal diameter of the shroud to be varied to match the external diameter of the blade tips, either as previously calculated for the engine condition in question or as directly measured using a transducer of some kind.
The designs evolved to meet this problem have tended to fall into two main categories, in one of which shroud diameter variation has been effected by mechanical means and in the other of which this has been effected by thermal means. The mechanical devices, while swift in operation have tended to be heavy, and/or complicated and the thermal devices have tended to be simpler but slower to react.